1. Field of the Invention
The present invention relates to a photographing device which has been used in a still camera, a video camera, a still video camera and so forth. The photographing device is employed in the above mentioned cameras to form optically an image of an object on a photosensitive element or an image pick-up device. In particular, the present invention relates to a photographing device in which a driving member is located outside a lens barrel and is arranged to drive a driven member in the lens barrel.
2. Description of Related Art
According to FIG. 10, in which a video camera is an example, a photographing device per se known, which is a zooming device having an automatic focusing function, is shown. A part of the prior-art photographing device is arranged in a lens barrel attached to the front side of the video camera. When a zooming switch 2, which is provided on a camera body 1, is manually operated, a driving motor 3 rotates forwardly or reversely, and a driving force generated by the motor 3 is transmitted to a cylindrical cam ring 5 through a gear train 4, so that the cam ring 5 consequently rotates around an optical axis of the lens barrel.
In the cylindrical cam ring 5, a frame 6 is fixedly located along the optical axis, and a pair of guide shafts 6a and 6b are arranged in the frame 6 in a manner that the shafts 6a and 6b are parallel with the frame 6. In the lens groups which constitute a photographing optical system, a front movable lens group 7 and a rear movable lens group 8 are arranged to be movable along the pair of guide shafts 6a and 6b. Specifically, a lens supporting frame 7A for the front movable lens group 7 and a lens supporting frame 8A of the rear movable lens group 8 are moveably engaged with the pair of the guide shafts 6a and 6b. Moreover, the front lens supporting frame 7A and the rear lens supporting frame 8A, respectively, have projections 7a and 8a projected in a radial direction of the cylindrical cam ring 5 so that these projections 7a and 8a engage with cam grooves 5a and 5b formed on the cylindrical cam ring 5.
When the cylindrical cam ring 5 is rotated by the motor 3, a zooming operation is performed by a movement of the lens supporting frame 7A (the front movable lens group 7) and the lens supporting frame 8A (the rear movable lens group 8) along with the cam grooves 5a and 5b and the pair of the guide shafts 6a and 6b. After a predetermined focal length is set, an automatic focusing is performed, so that an image of an object is formed on a CCD (charge coupled device) 9. In the camera body 1, a control circuit 10 is provided to control the motor 3.
In a photographing device of the type explained above, a zooming operation is performed by relative movement of the front and rear movable lens groups 7 and 8 with rotation of the cam ring 5. On the other hand, for performing focusing, a part of the lens group is required to be moved independently from the rest of the lens groups provided in the lens barrel. For the purpose of focusing, in the prior-art photographing device of FIG. 10, a fine adjustment of the front movable lens 7 is made by moving the front movable lens 7 forwardly or rearwardly with respect to the lens supporting frame 7A. An example of the fine adjustment of the front movable lens 7 for the purpose of focusing is shown in FIG. 11. An auxiliary motor 3A (an auxiliary driving source) exclusively used for focusing is provided on the lens supporting frame 7A, so that the lens supporting frame 7A can only be moved forwardly or rearwardly along the optical axis direction by the auxiliary motor 3A.
The arrangement of the auxiliary motor 3A as shown in FIG. 11 may cause adverse effects to the whole structure of the lens system, such as vibration of lenses, and blur of an image, because the auxiliary motor 3A inevitably generates vibration while the same is rotating, and the vibration is transmitted to the front movable lens group 7, the rear lens movable lens group 8, and other lens groups.
In addition to the above adverse effects, as the auxiliary motor 3A is provided at a circumferential portion of the lens supporting frame 7A, a weight-balance of the lens supporting frame 7A with respect to the optical axis is not in equilibrium, which generates a moment. Consequently, an uneven force is applied to the lens supporting frame 7A, so that smooth traveling of the lens supporting frame 7A is hampered. Due to a helicoid connection between the front movable lens 7 and the front lens supporting frame 7A, which is usually observed in this type of photographing device, if the uneven force is applied to the helicoid, repeated movement of the front movable lens 7 causes wear of the helicoid. These adverse effects may shorten the life expectancy and may degrade reliability of the photographing device.
The above explanations are based on a case in which at least one of the lens groups in the photographing optical system can be moved independently from other lens groups. The same type of problems mentioned above may possibly arise in a case where an aperture control mechanism and so forth, provided in a lens barrel, is operated independently, by an auxiliary motor provided in the lens barrel, from the lens groups of the photographing optical system.